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====== OpenCV vs matplotlib ======

data format
  - both matplotlib and opencv express the image as a numpy array
  - you can always manipulate the array directly using matrix math
  - or, both matplotlib and opencv provide methods to do sophisticated operations
  - there is considerable overlap between the two systems
  - you can probably do whatever you want with either system
  - and you can mix and match both systems, so long as you know the differences 
  - in the way they handle the array
color model

  - by default, matplotlib uses RGB, opencv uses BGR

alpha channel
  - by default, array shape is (y,x,3)
  - can be changed to (y,x,4), for RGBA or BGRA 
  - example:
    - y,x,d = myarray.shape
    - numpy.dstack(myarray, numpy.zeros(y, x))
    - [[https://stackoverflow.com/questions/39642721/adding-alpha-channel-to-rgb-array-using-numpy]]

display
  - by default, matplotlib produces a graph
  -   - with x,y,z axis, with tickmarks and scale
  -   - the graph is positioned with margins inside a resizeable window
  - opencv gives a full-size image in a fixed-size window

animation
  - both systems provide systems for animation and user-input handling
  - matplotlib FuncAnimation allows for an incremental blit 
  - matplotlib allows you to change the data of objects already in the plot
  - opencv requires you to rewrite the whole screen

user input
  - both systems allow you to wait for a key press
  - both systems provide an event-handler for keyboard and mouse events

tiling
  - numpy hstack() and vstack() can be used to tile multiple images into one
  - in addition, matplotlib uses the Figure->Axes->Plot heirarchy of subplots

how to overlay transparent plot on top of a photo

1. convert plot to image  
google: convert matplotlib figure to numpy array opencv  
https:  www.autoscripts.net/convert-matplotlib-figure-to-cv2-image  /

2. overlay transparent plot on top of image  
https:  docs.opencv.org/3.4/d5/dc4/tutorial_adding_images.html  

in cv2,  
use cv2.inRange() to make a mask  
use cv2.bitwise_and() to make masked image  
see ../sk8/visualcortex.py  

in matplotlib,  \\
use imshow() twice, where second, top, image has alpha channel\\  
https://stackoverflow.com/questions/49025832/combine-picture-and-plot-with-matplotlib-with-alpha-channel \\
ax.imshow(bottom, interpolation=None) \\ 
ax.imshow(topimg, interpolation=None) # top image must have alpha channel  \\